This invention relates to the art of improving heat transfer from heated surfaces to boiling liquids, and particularly to surfaces which enhance the phenomenon of nucleate boiling. The invention also relates to a method for forming a layer containing the surfaces.
The transfer of heat at effective rates from a heated surface to a boiling liquid in contact therewith ordinarily requires a substantial temperature difference between the surface and the liquid which greatly affects the efficiency of heat transfer. One important factor controlling this efficiency is the nature of the heated surface in contact with the liquid; it being known, for example, that smooth boiling surfaces produce low heat transfer coefficients on the boiling side. Low boiling heat transfer coefficients often severely restrict the heat transfer capacity of boiling apparatus. For example, when the heat for boiling is supplied by a vapor condensing on a smooth-walled heat transfer surface, the condensing heat transfer coefficient may easily be on the order of 2,000 B.t.u./hr./sq. ft./.degree.F., while the boiling heat transfer coefficient against the opposite side of the heat transfer surface may be only 100 to 200 B.t.u./hr./sq. ft./.degree.F. According to the familiar method of summing heat transfer resistances when the boiling and condensing heat transfer surfaces are of equal area, the overall heat transfer coefficient U is obtained approximately as follows: ##EQU1## where h.sub.B and h.sub.C are the boiling and condensing heat transfer coefficients respectively. It is clear that if h.sub.B is small compared to h.sub.C, then the value of U approaches h.sub.B and most of the advantage of a high condensing coefficient is lost.
Principal objects of this invention are: to provide a thermally conductive wall for transferring heat to a boiling liquid in a heat exchange apparatus having a boiling surface layer containing a plurality of cavities adapted to provide boiling nucleation sites within the surface layer; to provide a thermally conductive wall with a grooved boiling surface layer of a character which produces boiling heat transfer coefficients many times as large as those obtained with conventional smooth or roughened surfaces; and to provide a cross-grooved boiling surface layer of a character that is able to transfer to a boiling liquid large quantities of heat at much lower temperature differences than required in conventional heat exchange apparatus.
These and other objects and novel features of the invention will become apparent from the following description and accompanying drawing.